BF4 and SLI update:
http://www.bit-tech.net/hardware/graphics/2013/11/27/battlefield-4-performance-analysis/5
*Just so it's clear, in SLI each GPU reads/writes to it's own set of VRAM. The output just ALTERNATES (Frame#1 from GPU#1, Frame#2 from GPU#2...).
If we assume at least 80% scaling (probably better but so many patches..) then you can simply take the GTX780 benchmark and multiply the score by 1.8x.
For example:
At 5760x1080, the HIGH PRESET gives you 38FPS average (24FPS LOW). So 2xGTX780's at 80% scaling give about 68FPS.
So there's no problems with VRAM amount provided you don't crank everything to the absolute max, but then that's always been true of PC gaming. There's a balance.
MANTLE:
Mantle is interesting, but selling your current card and spending $1000 or so to get AMD cards just to support Mantle seems like a waste of money to me.
For one thing, you can get over 60FPS with 2xGTX780 on HIGH. Plus, this is really just one game anyway. Keep in mind SLI is better on average than Crossfire for all the other games you play.
Mantle is also just been released for BF4 so it's hard to predict exactly what benefit you'd get. It seems to be more about getting rid of the CPU bottleneck currently. Again though, how much MONEY are you willing to spend to get ULTRA at 60FPS when HIGH plus a few higher tweaks can maintain 60FPS?